Reduction Reactions of Electrolyte Salts for Lithium Ion Batteries: LiPF₆, LiBF₄, LiDFOB, LiBOB, and LiTFSI

Abstract

The reduction products of common lithium salts for lithium ion battery electrolytes, LiPF 6 , LiBF 4 , lithium bisoxalato borate (LiBOB), lithium difluorooxalato borate (LiDFOB), and lithium trifluorosulfonylimide (LiTFSI), have been investigated. The solution phase reduction of different lithium salts via reaction with the one electron reducing agent, lithium naphthalenide, results in near quantitative reactions. Analysis of the solution phase and head space gasses suggests that all of the reduction products are precipitated as insoluble solids. The solids obtained through reduction were analyzed with solution NMR, IR-ATR and XPS. All fluorine containing salts generate LiF upon reduction while all oxalate containing salts generate lithium oxalate. In addition, depending upon the salt other species including, Li x PF y O z , Li x BF y , oligomeric borates, and lithium bis [N-(trifluoromethylsulfonylimino) A typical lithium-ion battery contains a graphite anode, a lithiated transition metal oxide cathode, and an electrolyte solution composed of inorganic lithium salts dissolved in a mixture of organic carbonate solvents which frequently includes electrolyte additives.1 The longterm cyclability of the lithium-ion battery is dependent upon the anode solid electrolyte interphase (SEI), formed due to the electrochemical reduction of the electrolyte solution.2 Understanding the mechanisms of the reduction reactions along with the products of the reactions is essential for the development of better lithium-ion batteries. The SEI has been proposed to contain lithium alkyl carbonates, lithium carbonate, lithium oxalate, lithium alkoxides, and lithium oxide from the carbonate solvents and LiF, lithium fluorophosphates, lithium fluoroborates, and lithium oxalate from the reduction of electrolyte salts, depending upon the salt utilitzed.3-21 Electrolyte additives have also been used to tailor the properties of the SEI through preferential reduction on anode.1 Despite significant effort over the last two decades, the formation mechanism of the SEI is not well understood. One difficulty in understanding the composition of the SEI is that the SEI is a complicated mixture of compounds, which results from multiple simultaneous and competing reduction reactions. In addition, since the SEI is very thin (∼ 50 nm) and unstable in the presence of oxygen or water, characterization is very difficult. We have reported a detailed analysis of binder free graphitic anodes cycled in simplified electrolytes which suggest that the initial reduction reaction of the carbonates generate lithium alkyl carbonates and LiF as the predominant components of the anode SEI.20,21 Synthesis of initial SEI components from carbonate solvents in high yield through reduction of the solvents with lithium naphthalenide has been reported. The unique advantage of this reduction technique is the generation and isolation of SEI constituents from individual electrolyte components in high yield without competing reduction reactions. Reduction of ethylene carbonate re...

Customizing a Li–metal battery that survives practical operating conditions for electric vehicle applications

“…These cross-linked oligomeric borates are believed to render the SEI layer elastic and electrically conductive. 19 Hence, the reduction of the LiDFOB additive induced the emergence of organic-inorganic complexes that effectively passivated the Li metal surface. 20 To further assess the effect of LiDFOB on the cycling stability, average Li coulombic efficiency (CE) values were determined from Li/Cu cells using the method reported by Zhang et al 21 The average CEs for EF-31 and EF-31-D were found to be 94.6% and 98.1%, respectively; this result confirms that the addition of LiDFOB into EF-31 improves the stability and electrical conductivity of the SEI layer formed in situ (Fig.…”

Using Triethyl Phosphate to Increase the Solubility of LiNO3 in Carbonate Electrolytes for Improving the Performance of the Lithium Metal Anode

“…15,16 The results suggest an importance of the relative reduction reactions of the solvents and the salts in SEI formation and stability. 17,18 These results lead to the investigation of electrolytes containing LiDFOB, LiNO 3 and organophosphates.…”

Toward Critical Electrode/Electrolyte Interfaces in Rechargeable Batteries

“…The altered SEI components and structures greatly influence the Li deposition morphology. [ 117 ] Lithium bis(fluorosulfonyl)imide (LiFSI) becomes an attractive Li salt recently due to its superior compatibility toward Li metal anode. [ 118 ] SF bond is found to more easily undergo electrochemical cleavage at higher potential than the other XF bonds, including PF in PF 6 − and CF in TFSI − , inducing the formation of LiF component.…”